Added FreeBSD w/ ZFS performance instructions
Added FreeBSD w/ ZFS performance instructions

On FreeBSD systems with ZFS, sendfile() is useless as ZFS's caching subsystem (adaptive replacement cache a.k.a ARC) already caches most frequently used files in RAM. Disabling sendfile() avoids redundant data caching. 

References: 
https://calomel.org/nginx.html
http://blog.vx.sk/uploads/conferences/EuroBSDcon2012/zfs-tuning-handout.pdf
Olvikolvi/server-configs-nginx,eric-seekas/server-configs-nginx,jlien-calwater/server-configs-nginx,Qoto/server-configs-nginx,yshaojie/server-configs-nginx,dliggat/sample-nginx-config,k0nsl/server-configs-nginx,susaing/server-configs-nginx,bond-agency/server-configs-nginx,georgemarselis/server-configs-nginx,cxnam/server-configs-nginx,TheBookPeople/server-configs-nginx,kane-c/server-configs-nginx